On Tue, Feb 27, 2024 at 09:35:39AM -0600, Eric W. Biederman wrote: > Kees Cook <keescook@xxxxxxxxxxxx> writes: > > > On Tue, Jan 23, 2024 at 12:23:27AM +0900, Jan Bujak wrote: > >> On 1/22/24 23:54, Pedro Falcato wrote: > >> > Hi! > >> > > >> > Where did you get that linker script? > >> > > >> > FWIW, I catched this possible issue in review, and this was already > >> > discussed (see my email and Eric's reply): > >> > https://lore.kernel.org/all/CAKbZUD3E2if8Sncy+M2YKncc_Zh08-86W6U5wR0ZMazShxbHHA@xxxxxxxxxxxxxx/ > >> > > >> > This was my original testcase > >> > (https://github.com/heatd/elf-bug-questionmark), which convinced the > >> > loader to map .data over a cleared .bss. Your bug seems similar, but > >> > does the inverse: maps .bss over .data. > >> > > >> > >> I wrote the linker script myself from scratch. > > > > Do you still need this addressed, or have you been able to adjust the > > linker script? (I ask to try to assess the priority of needing to fix > > this behavior change...) > > Kees, I haven't had a chance to test this yet but it occurred to me > that there is an easy way to handle this. In our in-memory copy > of the elf program headers we can just merge the two segments > together. > > I believe the diff below accomplishes that, and should fix issue. > > Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> > > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c > index 5397b552fbeb..01df7dd1f3b4 100644 > --- a/fs/binfmt_elf.c > +++ b/fs/binfmt_elf.c > @@ -924,6 +926,31 @@ static int load_elf_binary(struct linux_binprm *bprm) > elf_ppnt = elf_phdata; > for (i = 0; i < elf_ex->e_phnum; i++, elf_ppnt++) > switch (elf_ppnt->p_type) { > + case PT_LOAD: > + { > + /* > + * Historically linux ignored all but the > + * final .bss segment. Now that linux honors > + * all .bss segments, a .bss segment that > + * logically is not overlapping but is > + * overlapping when it's edges are rounded up > + * to page size causes programs to fail. > + * > + * Handle that case by merging .bss segments > + * into the segment they follow. > + */ > + if (((i + 1) >= elf_ex->e_phnum) || > + (elf_ppnt[1].p_type != PT_LOAD) || > + (elf_ppnt[1].p_filesz != 0)) > + continue; > + unsigned long end = > + elf_ppnt[0].p_vaddr + elf_ppnt[0].p_memsz; > + if (elf_ppnt[1].p_vaddr != end) > + continue; > + elf_ppnt[0].p_memsz += elf_ppnt[1].p_memsz; > + elf_ppnt[1].p_type = PT_NULL; > + break; > + } > case PT_GNU_STACK: > if (elf_ppnt->p_flags & PF_X) > executable_stack = EXSTACK_ENABLE_X; I don't think this is safe -- it isn't looking at flags, etc. e.g., something like this could break: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align LOAD 0x003000 0x12000 0x12000 0x001000 0x001000 R E 0x1000 LOAD 0x004000 0x13000 0x13000 0x000000 0x001000 RW 0x1000 Hmm -- Kees Cook
